The apparatus of the present invention is directed to measuring the salinity of a solution. In particular the salinity of solutions in a well borehole is measured in situ. As an example, either at the drilling stage or in later operation of a producing well, formations along the well borehole may produce substantial quantities of water. The measure of salinity of the water in the borehole is important. For instance, some of the formations being drilled may be fresh water, and other formations may produce salt water. It is particularly important when salt water is encountered to provide a means for isolation of salt water so that it does not travel up or down the borehole and invade nearby fresh water or petroleum bearing formations. The drilling fluid which is ordinarily used in the drilling process includes substantial portions of water.
In another circumstance, a producing well may produce oil above a salt water formation for a period of years. Typically, the drive or pressure in the producing formation forces the fluids upward. If the oil is depleted by draining the formation at a certain depth, the oil may stop flowing into the well and the driving water below may invade the well. In the instance where this invading fluid is salt water, significant changes in the operation of the well must be undertaken. For instance, the well may require plugging of the original perforations and reperforation at a higher elevation. Moreover, the impact of salt water (compared with fresh water) on the equipment located in the well is important. Salt water is more corrosive than fresh water. Damage to the equipment in part depends on the salinity of the water produced in the well. Some wells may produce a mix of oil and water and again, the salinity of the water produced by the well becomes important.
Perhaps the most important use of salinity measuring apparatus is in conjunction with other logging equipment which performs a TMD measurement capture cross section. The data obtained from such a measurement system is significantly aided if salinity of water is also furnished. Thus, the salinity measurement of this apparatus (obtained in the well) assists in delineating oil and water to infer a measurement of moveable oil. The salinity measuring device is very desirable as an adjunct to other measuring instruments.
For these reasons and numerous other reasons, such as an aid in the interpretation of well logs it is important to periodically measure the salinity of water or drilling fluid in a well.
This apparatus sets forth an in situ salinity measuring system. The system is particularly desirable for use in measuring salinity over a very wide range. Borehole waters may range in salinity from fresh water to greater than 26% by weight salt content thus, requiring a large dynamic range. Other salinity measurement techniques include measurement of electrical properties such as resistivity or conductivity, specific gravity and chemical analysis of the water. Substantially pure water is fairly resistive; saturated salt water is reasonably conductive. However, electrical measurements have a limited range and accurate measurement requires sample dilution or a change in calibration cell for salinities greater than about 10% salt. A specific gravity measurement using hydrometers gives rise to some problems in use and handling. The range of one hydometer is limited, and hence the full range of salinity cannot be accurately determined by one hydrometer. In fact, several would be required to determine the full range of salinity. Moreover, chemical analysis, hydrometer and the electrical measurements do not lend themselves to in situ use in a borehole. Therefore, these methods require some type of device to be placed into the borehole to bring the sample to the surface for delayed measurement.
With these limitations on other approaches in view, the present apparatus is directed primarily to a salt water concentration measurement procedure particularly adapted to be performed in a down hole environment. This yields an apparatus which can be mounted in a pressure resistant body member or sonde, lowered on an armored logging cable into a well to any selected depth. The present apparatus utilizes an elongate measuring chamber within the sonde. Suitable perforations in the wall of the sonde defining the chamber are incorporated to enable well fluid to flow into and out of the chamber. The measuring chamber has upper and lower ends. A reflector is placed at one end and an acoustic transducer is placed at the opposite end. An acoustic wave is transmitted through the liquid and bounced off the reflector surface opposite it. The return signal is observed utilizing an acoustic transducer. The elapsed time of travel is noted by a time measuring circuit. Changes in transit time are proportional to changes in salinity. Thus, with a fixed spacing between the transducer and the reflector surface a norm is established for water having no salt. The changes in transit time yield a measure of salinity.
This apparatus is particularly useful in determining the measure of salinity over the wide dynamic range up to full saturation.